1. Field of the Invention
The present invention relates to a cellulose acylate film and its production method, and to a polarizer and a liquid crystal display device comprising the cellulose acylate film. In particular, the invention relates to a cellulose acylate film favorable for use as an optical film such as a polarizer protective film, an optical compensatory film, etc.
2. Description of the Related Art
With the recent tendency toward advancing TV use of liquid crystal display devices, the panel size of the devices is enlarged and high-definition and low-price liquid crystal display devices are much desired. In particular, VA-mode liquid crystal display devices have a relatively high contrast and enjoy a relatively high production yield, and are therefore most popular liquid crystal display devices for TV use.
However, VA-mode liquid crystal display devices have a problem in that, at the time of black state, the devices could provide black that is good in some degree in the normal direction to the display panel, but when the black state panel is seen in viewing angle directions (oblique directions), there occurs light leakage to disable background black display whereby the viewing angle is narrowed. Accordingly, a retardation film is desired capable of expressing a retardation level in such a degree that enables viewing angle compensation.
Recently, further, for preventing the neutral tone on a liquid crystal display panel from being yellowed, a multigap (MG) cell has become used in which the thickness of the liquid crystal layer, or that is, the cell gap is changed for every color. However, the multigap cell is problematic in that, as compared with that on a conventional liquid crystal display panel, the color shift at the time of black state in viewing angle directions increases, and therefore, it has become much desired to further improve the multigap cell in point of preventing the color shift at the time of black state in viewing angle directions on a liquid crystal display panel.
Further improvement of the display performance of liquid-crystal display devices and further reduction in the production cost thereof are still desired, and it is desired to provide an inexpensive retardation film capable of fully attaining viewing angle compensation even though it is thin, and capable of enabling further contrast increase.
Regarding the requirements, it is known that use of a retardation film having reversed wavelength dispersion characteristics of retardation, or that is, a retardation film having optical properties of such that its in-plane retardation Re increases on a longer wavelength side is effective for preventing color shift at the time of black level of display in viewing angle directions on a liquid-crystal display panel (see Patent Reference 3).
Films having reversed wavelength dispersion characteristics of retardation that have heretofore been investigated are produced by adding an additive having a negative intrinsic birefringence to a resin film. However, the additive having a negative intrinsic birefringence is expensive and has some problems in that, when such an additive having a negative intrinsic birefringence is added to a resin film, then the thickness-direction retardation Rth of the film lowers and therefore, in order to make the film express a desired retardation level, the thickness of the film must be increased or the amount of the retardation enhancer to be added to the film must be increased, and as a result, from the viewpoint of the material cost, the additive is unsatisfactory.
As opposed to this, Patent Reference 1 discloses a technique of adding an acrylic polymer having a negative intrinsic birefringence and further a sugar ester compound to a phthalyl/acetyl-modified heterogeneous cellulose-mixed cellulose acylate, thereby improving the light leakage resistance, the color shift resistance, the front contrast, the retardation and the wavelength dispersion characteristics of retardation of the resulting film.
On the other hand, there is known a method of improving various properties of cellulose acylate film by changing the stretching condition for the film. For example, Patent Reference 2 describes a method of increasing the water content of a cellulose acylate film by once drying the formed film and thereafter wet-treating the film. The patent reference says that, in drying, the film is preferably stretched by from 1 to 20% or so in the film width direction (in the direction transverse to the film traveling direction) at from 80 to 150° C. or so. However, the patent reference says nothing relating to use of a sugar ester compound.
Patent References 3 and 4 describe a method of stretching a film while the residual solvent amount therein is made to be 0%, 15% or 40% at the start of film stretching thereby to enhance the optical expressibility of the film or reduce the influence of humidity change on the film.
With the recent increase in the demand for slate PC and the like, thinner and lighter displays are desired, and thinner retardation films are also desired. However, for example, Patent Reference 1 discloses a cellulose ester film having a thickness of 50 μm.